Video from 2017. The age of the Carrowmore megalithic cemetery varies from source to source.
"The advent of radiocarbon dating has forced archaeologists to re-think the entire the chronology of western European megalithic culture. Prior to radiocarbon dating, it had been believed that megalithic culture had been brought to western Europe from the Mediterranean region, however, this was proven to be utterly false when C14 dating methods showed that the megalithic sites of western Europe substantially predated those of the Mediterranean. Not only that, but the monuments of western Europe were found to predate those of Egypt and the Near Eastern cultures also, which had previously been thought to have been the inspiration for the European megalithic builders." Source: Newgrange.
"The earliest dates of construction for western European megaliths are found to be in the west of Ireland. These early dates were first put forth in the late 1970's early 1980's by a group of Swedish archaeologists led by Stefan Bergh excavating in and around the Carrowmore megalithic complex. Initially, the very early carbon dates produced were met with some scepticism. The accuracy of Swedish teams findings were questioned as they seemed to shatter preconceived notions of the progression of megalith building in Europe. With time however, the megalithic sites of western Ireland have been proven to indeed be the oldest such sites in Europe."
Comment: I have no idea what to make of this theory. So far I haven't been able to find out how old these Swiss archaeologists thought the Carrowmore tomb complex is. According to Wikipedia, the site dates back to around 3700 BC, "making Carrowmore ... among the oldest cemeteries of megalithic tombs in Ireland." By comparison, the general consensus is that Stonehenge was constructed somewhere between 3000 and 2000 BC.
Note: According to the Megalithomania video above, Carrowmore is 7,000 years old, but the dating in Wikipedia would make it around 5700 years old.
I was under the impression that the megaliths on Malta were the oldest in Europe, but that may not be the case. The Ggantija megalithic temple complex on the Mediterranean island of Gozo are "the earliest of the Megalithic Temples of Malta." (Wikipedia) They're older than the Egyptian pyramids, but they still seem to date back to around the same time (~3600 - 2000 BC?).
It appears that all of these structures were built during the same general period, so it's possible, I guess, that the megalithic culture in Europe could have originated in Ireland. Unfortunately for the theory, a circle of stone pillars at Gobekli Tepi in Turkey, dating back to 10,000 - 8,000 BC, is currently thought to be the oldest megalith in the world. If that's accurate, then it would appear that these fantastic stone structures originated in the East and then spread into Europe. That's the general consensus, anyway.